1. Field of the Invention
The present invention generally relates to an integrated circuit, and more particularly to a coplanar transformer with a capacitor.
2. Description of the Prior Art
The transformer is an electromagnetically coupling device using two coils of wire, from one coil an AC signal being inputted to make another coil produce a corresponding AC signal. Generally speaking, the former one is called primary coil, and the latter one is called secondary coil. The efficiency of electromagnetically coupling is the efficiency of transferring of energy from primary coil to secondary coil, in which the sensitivity of transformer to noise in whole is also contained.
The two coils in transformer are deployed as being wrapped in a core part, which can be air core or ferrite core used to improve coupling efficiency. And the deployment of coils opposite to core part are stacked, one wrapped in front and one in back, or interleaved.
The transformer in which the coils are deployed as stacked or interleaved can be used in Multi-Layer IC Circuits, in which the stacked coils being conductors of two-layer spiral structure weaved one coil up and one coil down, the interleaved coils being two coils of conductors inter-woven on conductor pattern of a coplanar spiral structure, and the two having no contact with each other. High capacitance occurs when the coils are staked, meaning that the coils in one coil up and one coil down structure will be like two electrodes of capacitor. This capacitive coupling will cause phase-shifts and amplitude errors. Compared with staked coils, in transformer with coplanar interleaved coils, conductors of each coil are set next to each other rather than staked on top of each other. Therefore capacitive coupling is decreased and there will be fewer phase-shifts and amplitude errors.
FIG. 1A and FIG. 1B are diagrams that show the back of a coplanar interleaved transformer. This transformer has primary coil 110 and secondary coil 120, end point 111 and end point 112 being end points of primary coil 110 and end point 121 and end point 122 being end points of secondary coil 120. Primary coil 110 and secondary coil 120 are constituted respectively by several conductor segments 140 and connected conductors 130, all conductor segments 140 being located at a first metal layer and all connected conductors 130 being located at a second metal layer. Wherein, two ends of each connected conductor 130 are respectively electronically coupled with a conductor segment 140 for winding round a conductor segment 140 of another coil to keep primary coil 110 and secondary coil 120 interleaving on the same plane from contacting each other. Obviously, coplanar interleaved transformer and staked transformer need more square measure but can avoid capacitive coupling more efficiently.
Transformer is generally used for providing electric isolation between signals and can be used for coupling a signal with a resonant L-C circuit, making one of the coils provide inductance or part of inductance. And typical radio-frequency transmitters contain this kind of RF transformer that combines resonant antenna stage and outputting stage. As shown in FIG. 1C, when primary coil 110 is in parallel connection with a capacitor 150, compared with prior design, the transfer of energy from primary coil 110 to secondary coil 120 can be increased. Thus increase of efficiency of transfer or improvement of matching can maximize the integral yield. This becomes very important when the energy inputted from primary coil 110 is very small (for example, signal received by antenna) and the integral efficiency can therefore be improved. However, extra capacitor device means need of extra space. Concerning this coplanar interleaved transformer that has extravagant use of space in structure, if the capacitor and transformer can be combined in the same space, many costs can be saved.